1. Field of the Invention
The present invention relates to a mirror holder for adjusting the optical axis of an optical communication device and to an optical axis correcting device using the same.
2. Description of the Related Art
Nowadays, due to the shortage in radio wave resources and the requisite complicated procedures for installing wireless or wire communication devices, researches are being assiduously conducted for putting optical communication into practical use.
However, no device has been realized yet which provides a sufficient performance in long distance optical communication in the order of kilometers.
A conventional example of an optical communication device which is capable of long-distance two-way optical communication will be described with reference to FIGS. 9 through 11. FIG. 9 illustrates the optical system of a conventional optical communication device; FIG. 10 illustrates the optical axis adjusting method for this optical communication device; and FIG. 11 illustrates another optical axis adjusting method.
First, the operation of the transmission system will be described. As shown in FIG. 9, a beam from a semiconductor laser 41 modulated on the basis of a transmission signal is converted to a parallel beam by a lens 42 and impinges upon a beam splitter 43. The beam from the beam splitter 43 impinges upon a concave lens 44 in the output direction, and is enlarged by the lens 44 before it is converted to a substantially parallel beam by a main lens 45. The beam is then output as transmission beam Lout toward the optical communication device at the other end (not shown).
Next, the operation of the reception system will be described. The reception beam Lin from the optical communication device at the other end is received by the main lens 45 and converted to a parallel beam by the lens 44. It is then divided by a beam splitter 46 into a beam in the direction of a position detection sensor 48 condensed by a lens 47 and a beam in the direction of a photoreceptor 50 condensed by a lens 49. The output from the photoreceptor 50 is demodulated as the reception signal to restore the signal.
In the optical communication device having the above-described optical system, it is necessary for the optical axis on the transmission side to coincide with the optical axis on the reception side. However, the optical system unit is influenced by external factors such as wind, vibrations generated inside the device, changes in the ambient temperature, etc., and a deviation in the optical axes is generated. In long distance communication, a minute deviation in the optical axes interferes with the communication, so that it is necessary to perform correction to make the optical axes coincide with each other.
Various conventional methods for compensating for this optical axis deviation have been proposed.
One of these conventional methods adopts the construction as shown in FIG. 10, in which the above-described optical system is integrally accommodated in a lens barrel 61. The lens barrel 61 is supported by an intermediate ring 62 by means of upper and lower X-axis bearings 65 so as to be rotatable in the X-axis direction. The rotation of an X-axis motor 63 fastened to the intermediate ring 62 is transmitted to a driven gear 66 fastened to the X-axis bearing 65 through the intermediation of a driving gear 64 to rotate the lens barrel 61 in the X-axis direction. Further, the intermediate ring 62 is supported on a seat 71 by right and left Y-axis bearings 69 so as to be rotatable in the Y-axis direction. The rotation of a Y-axis motor 67 fastened to the seat 71 is transmitted to a driven gear 70 fastened to the Y-axis bearing 69 through intermediation of a driven gear 68, and the intermediate ring 62, in other words, the lens barrel 61, is rotated in the Y-axis direction. The X-axis motor 63 and the Y-axis motor 67 are controlled on the basis of the detection output of the position detection sensor 48 of the above-described optical system, and are controlled so as to effect coincidence in optical axis between the devices.
However, in the above-described construction, the lens barrel 61 as a whole is used as an operating section, so that the inertial mass is large. Thus, it is rather poor in control responsiveness. Further, it requires a high-accuracy, highly rigid bearing and a motor with large driving power. Further, since the optical axis adjustment has to be performed in minute angles, the motor and the torque transmission mechanism must be ones of high accuracy which are free from backlash.
FIG. 11 shows another example. In this example, there are adopted a pair of correcting sections which are arranged perpendicular to each other in the optical path: an X-axis mirror 81 and an X-axis motor 82 for rotating this, and a Y-axis mirror 83 and a Y-axis motor 84 for rotating this. To perform angular correction of each mirror, the X-axis motor 82 and the Y-axis motor 84 are rotated to control the X-axis mirror 81 and the Y-axis mirror 83, constantly effecting coincidence in optical axis between the devices.
However, in this method also, a mirror and a motor are required for each optical axis correcting direction, with the result that the construction of the optical axis correcting device is complicated. Further, as in the above-described example, the motor and the mechanism section for transmitting torque must be highly accurate ones free from backlash.
As a means for solving the above problem, the present inventors have proposed an optical axis correcting device, which is described in Japanese Patent application No. 10-14533. In this optical axis correcting device, an optical axis correcting mirror is held at the center of a mirror holder (hereinafter referred to as "2-axis spring") so as to be independently rotatable around two axes on the mirror surface. The 2-axis spring has three concentric annular sections formed of thin elastic rolled plates, with connecting sections being provided between the annular sections so as to allow torsional rotation.
However, generally speaking, a rolled member exhibits different physical characteristics between the rolling direction and a direction perpendicular thereto, the control characteristics of the two axes differing depending upon the positional relationship between the two connecting sections and the rolling direction. Further, the metal material exhibits a small internal loss attenuating vibration, and once vibration is generated, it is hard to attenuate.